How can i trigger a script via player, and not NPCs?

I'm an early scripter, but i wanted to know how you could detect a player touch using scripts. I'm making a game with ghosts in which chase you, and when touched, they die and respawn. The problem is that I include other NPCs in my game which also trigger the script. Here is the script i am using...

ghost = script.Parent.Parent

function onTouched(hit)
    local b = hit.Parent:FindFirstChild("Humanoid")
    if b ~= nil then
ghost.Humanoid.Health = 0
	end
end
script.Parent.Touched:connect(onTouched)

Also, the script being used is contained insided each humanoid 'LeftArm','UpperperTorso', 'Head' etc.

Any ideas on how to fix this?

You can use if statement to check if the one that touch it is a player

ghost = script.Parent.Parent

function onTouched(hit)
  if game.Players:FindFirstChild(hit.Parent.Name) then
  	local b = hit.Parent:FindFirstChild("Humanoid")
  	  if b ~= nil then
		ghost.Humanoid.Health = 0
  	  end
  end
end
script.Parent.Touched:connect(onTouched)